 National Advisory Committee for Aeronautics

The National Advisory Committee for Aeronautics was the United States' aeronautical research organization established in 1915. Its laboratories and personnel formed the core of the National Aeronautics and Space Administration in 1958. Its technical legacy includes wind tunnels, systematic NACA airfoil families and published measurements that let designers compare aircraft wings.^[ref]

A particularly useful research strategy was to change geometry systematically while keeping test conditions comparable. Eastman N. Jacobs, Kenneth E. Ward and Robert M. Pinkerton varied airfoil thickness-to-chord ratio and airfoil camber in their related-section experiments, rather than testing only unrelated existing designs. The measurements connected an explicitly specified shape with lift coefficient, drag and pitching moment.^[ref]

An original similarity calculation illustrates why the apparatus mattered. A small model with airfoil chord $0.10\mathrm{m}$ in air of density $1.2kg/{\mathrm{m}}^3$, dynamic viscosity $1.8\times 10^{-5}\mathrm{Pa}\mathrm{s}$ and speed $30m/s$ has Reynolds numberA five-times-larger chord at the same conditions has $Re=10^6$. A variable-density wind tunnel can instead raise the model's air density fivefold to match that Reynolds number, approximately holding dynamic viscosity fixed. Matching it alone does not also establish matching Mach number, surface condition or wind-tunnel turbulence intensity.

The enduring contribution is a reusable experimental comparison: specify shape, operating conditions and how forces were measured, then publish results that can be checked against another experiment or computational fluid dynamics calculation.